Wetting properties of a surface of a material are dependent on the chemical composition or geometrical structures of the material surface. Generally, the surface wetting properties of a material can be determined by measuring the contact angle of a liquid droplet on the surface. When water is used, a small contact angle (ie less than 90°) is indicative of a hydrophilic surface while a large contact angle (ie more than 90°) is indicative of a hydrophobic, surface. For example, a hydrophilic surface such as glass exhibits a contact angle with water in the range of 5° to 25° while poly(dimethylsiloxane) exhibits a contact angle of 109° and is hydrophobic.
By measuring the contact angle from different directions, the isotropic or anisotropic properties of surface wetting can be determined. If the contact angles are the same when measured from different directions, the wetting property is isotropic. If the contact angles are different when measured from different directions, the wetting property is anisotropic.
Scientists and engineers have developed various methods to tailor the surface wetting properties of a material by altering the contact angle. By mimicking structures found in nature, certain desired wetting properties can be achieved. For example, the surfaces of lotus leaves are superhydrophobic due to the presence of micro/nanoscale hierarchical structures which allow water droplets to roll off easily, taking dirt and contaminants with them, leading to a self-cleaning effect. Such an effect is desirable in paints, roof tiles, fabrics or any other surfaces where self-cleaning is needed.
Generally, surface wetting properties are modified either through chemical or physical means. Roughening a surface can greatly enhance the hydrophobicity or hydrophilicity of the surface.
Some chemical surface wetting modification methods include fluorination, formation of pores or adhesion of organic films to inorganic substrates. However, these methods may result in detrimental changes to the material, such as the material's mechanical integrity. Additionally, changes in the surface wetting properties may not be permanent.
On the other hand, physical means include plasma-enhanced chemical vapour deposition, ion-beam etching, microcontact printing and photolithography. However, complicated equipments are often required and led to unnecessary restriction of sample sizes.
When roughening a surface results in anisotropic patterns or roughness geometry, a liquid droplet shows a non-spherical shape when placed on such surfaces. The apparent contact angles of the droplet observed in directions perpendicular and parallel to the droplet are different, resulting in anisotropic wetting or de-wetting. Anisotropic wetting had been observed on chemically patterned liquidphilic surfaces and micropatterned monolayer surfaces with alternating liquidphilic/liquidphobic area prepared by vacuum ultraviolet photolithography. Anisotropic wetting properties have also been demonstrated on the surface of rice leaves leading to the replication of the rice leaf structure by growing aligned carbon nanotubes. However, efforts to tune the degree of anisotropic wetting have been met with limited success.
There is a need to provide a method to alter the surface wetting properties of a substrate that overcomes or at least ameliorates one or more of the disadvantages described above.